Oak (Quercus) is a versatile plant that provides shade, acts as a windbreak and produces acorns, which, in turn, feed wildlife. This large genus consists of evergreen and deciduous trees and shrubs. Depending on the species, the trees grow in U.S. Department of Agriculture plant hardiness zones 3 through 10. The mighty oak is subject to several diseases, some of which are nothing more than an annoyance, while others can threaten the life of the oak.
Anthracnose is a fungal disease that causes black spots with a tarlike appearance on the foliage of oak trees. Other symptoms of anthracnose are curled, distorted leaves, cankers, die-back and girdling. Anthracnose is prevalent in wet, humid conditions, and no chemical treatment is available. There are, however, steps you can take to control the branches and lessen the damage. As soon as diseased and damaged branches are spotted, remove and destroy them. Applying a fertilizer in the spring will help give the infected oak tree a boost. When applying the fertilizer, wait until the rainy season has ended and the oak leaves have opened.
Oak Branch Die-back, Leaf Blister and Twig Blight
Oak branch die-back affects various types of oak trees including English and black oak. It causes discolored leaves that wilt, branch die-back and bark peeling. Oak leaf blister -- also called leaf curl -- occurs when the leaves are flush and the foliage is wet. The leaves infected with leaf blister curl, gall and drop. Oak twig blight causes the twigs and leaves to discolor while remaining on the branch. It generally affects the current season’s growth and is most often seen in coast live oak. Oak branch die-back, leaf blister and twig blight can all be controlled with proper care. Water the oak only when adequate rainfall is not available and prune damaged or diseased branches and leaves off the oak.
When the oak tree begins to drip a clear or brown liquid from acorns, twigs or leaves, it is probably infected with the bacterial disease drippy oak. It generally occurs when the oak has wounds from pests such as gall wasps or filbertworms. But common garden pests that produce honeydew, like scales and aphids, will also cause drippy oak. Thankfully, drippy oak does not threaten the life of the tree and the liquid can be washed away with water. The only control for drippy oaks is to keep pests that are known to cause this bacterial disease at bay. Wetwood is caused by either bacteria or yeast organisms and produces areas of stained wood that seep fluid, cause foliage wilt and lead to stem die-back. Wetwood is generally not harmful to the life of the oak and can be controlled by exposing small wounds to air by opening them. This will also prevent liquids from accumulating. Drilling a hole into the infected tree and inserting a copper tube will drain the fluid, release gases and prevent the bacteria or yeast from spreading.
Rots and Rusts
Rots will occur when the oak is planted in overly soggy or poorly drained soil, leading to leaf discoloration, twig die-back, leaf dropping and clusters of brown, yellow or cream-colored mushrooms growing at the base of the trunk. No real treatment of rot is available, and preparing the site before planting the oak is the best defense against this fungal disease. Another class of fungal diseases affecting oaks are rusts. Symptoms of rust include cankers, leaf spots, branch die-back, galls, tissue swelling and leaf dropping. Spores and pustules will also begin to grow on leaves. Rust diseases are harmful to the life of the oak only in extreme cases, and applying a fungicide to the tree in the spring will help prevent fungus such as a rust from attacking the oak.
- University of California Statewide Integrated Pest Management Program: Oak—Quercus spp. Family Fagaceae (Beech Family)
- Neighborhood Association of Southwestern Williamson County: Oak Tree Diseases
- The University of Georgia College of Agricultural and Environmental Sciences: Key to Diseases of Oaks in the Landscape
- University of California IPM Online: Drippy Oak (Drippy Acorns)—Erwinia Quercina
- University of California IPM Online: Wetwood or Slime Flux
- University of California IPM Online: Rusts
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